The present invention concerns a method of operating a time-of-flight secondary-ion mass spectrometer for the purpose of analyzing mass spectra wherein several finely structured ranges of mass appear in isolation and widely separated, whereby
a) the surface of a sample of material is bombarded at regular intervals (cycle times t.sub.z) with primary-ion pulses,
b) secondary ions of different mass are thereby released from the surface and are accelerated to the same level of energy,
c) their mass-dependent time t of flight over a path 1 is measured and their mass determined therefrom.
Time t of flight is proportional to the mathematical square root of the mass (t proportional .sqroot.m) in this situation. The number of secondary ions equivalent to a particular mass m yield within a specified cycle time t.sub.z fine-structure maxima within "nominal ranges". Each nominal range corresponds to a whole-number atomic or molecular weight of elemental or molecular ions. The amplitudes of the fine-structure maxima allow qualitative and quantitative analyses of the composition of the sample's surface.
Time-of-flight secondary-ion mass spectrometer (TOF-SIMS) is known (from e.g. Analytical Chemistry 64 (1992), 1027 ff and 65 (1993), 630 A ff). It is employed for the chemical analysis of solid surfaces.
The surface of a sample is bombarded with a pulsed beam of primary ions at a pulse duration t.sub.p. The beam releases secondary ions from the surface. The free secondary ions are accelerated to the same level of energy E (a few KeV) in an extraction field and then travel along a flight path 1. At the other end of the path they are detected by a time-resolving detector. The great majority of secondary ions are simply charged.
The secondary ions' time of flight can be represented by EQU t=1/v 1/.sqroot.2E.multidot..sqroot.m=k/m (1)
The precise mass of a secondary ion can accordingly be calculated at constant energy from the detected time t of flight.
The secondary ions are registered in accordance with the desired range of masses within a specific interval, cycle time t.sub.z, subsequent to the impact of a primary-ion pulse. From equation (1), EQU t.sub.z =k/m.sub.max ( 2)
where m.sub.max =the largest mass within the desired range. The next primary-ion pulses can impact the sample once cycle time t.sub.z has lapsed. Times of flight t are accordingly measured at a frequency of repetition f=1/t.sub.z. Very few secondary ions, typically 0.1 to 10, are released and detected per cycle. A mass spectrum of adequate dynamics over several orders of magnitude, meaning an adequate ratio between the highest and lowest intensities, can be obtained by accumulating the counting events over a large number of cycles. The measurements typically take 100 to 1000 seconds.
Both elemental and molecular ions are released from the surface of the probe. The precise mass of a secondary-ion species, which can be either elemental or molecular, equals the sum of its atomic weights. Since the individual atomic weights deviate slightly from integral values due to the binding energy of the atomic nuclei, each aforesaid nominal-mass range will be found on each side of an integral value. The precise masses of elemental and molecular ions differ only slightly. One example of a secondary-ion species is 27 u: aluminum.sub.+ : 26.99154 u: C.sub.2 H.sub.3.sup.+ : 27.023475 u. The various species of secondary ions can be separated and resolved into fine-structure maxima, that is, if the mass resolution is high enough, and elements and compounds can be detected separately. The separation of such species is an essential prerequisite for demonstrating traces of compounds and elements. The mass resolution m/DELTAm employed in time-of-flight secondary-ion mass spectrometry relates to the mass difference DELTAm at which a mass m can still be separated into fine-structure maxima at. It depends decisively on primary-ion pulse duration t.sub.p. Other factors involved in the separation are the resolution capacity of the time-of-flight analyzer and the time resolution of the detector and recording electronics. Improving these factors are not, however, an objective of the present invention.
Time-of-flight secondary-ion mass spectrometry is employed not only to analyze the composition of surfaces, but also allows the detection of lateral distributions of various elements and compounds at a high local resolution, in the sub-.mu. range. The beam of primary ions is for this purpose focused on a very small point and gridded over the sample by means of a deflecting method. In imaging spectrometry, a mass spectrum is obtained and evaluated for every point on the grid. A distribution image can then be generated from the results for a number of points on the grid (typically 356.times.256). In deep-distribution analysis, the sample can be abraded with the primary beam or by an additional source of ions and a depth distribution of the various species established by analyzing each successive surface.
The primary-ion pulse duration necessary for high mass resolution is only a few nanoseconds for a typical drift of approximately 2 m. The pulses are generated by an appropriate beam-pulsing procedure from a static beam deriving from a source of ions. The number N.sub.p of primary ions per pulse derives from the static current I.sub.p through the ion source and pulse duration t.sub.p in the form EQU N.sub.p =I.sub.p .multidot.T.sub.p /e (3)
wherein e is the elementary charge.
It will accordingly be evident that the number of primary ions per pulse will decrease with the length of the pulse. Consequently, more primary-ion pulses will be necessary to generate and detect the same number of secondary ions. This means that the measurement time will increase. The increased measurement time is a particular problem in the analyses of microscopically dimensioned areas with finely focused ion sources because the available ion-source currents I.sub.p are very small. The recording of spectra of higher dynamics, of lateral distributions, and of distributions in depth often result in measurement times of more than one hour to several hours.
Measurement times can be decreased at the state of the art only by prolonging primary pulse duration t.sub.p, which is accompanied by a loss in mass resolution, or by increasing the rate of repetition, which is accompanied by restrictions in the mass range that can be covered (cf. Eq. 2).